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Geotechnical News • December 2014
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GEOSYNTHETICS
in studies conducted by different
investigators, over a number of years,
with occasional consideration given to
aspects of theoretical analysis.
Geotextile filter: origins of
current practice
Consider now the companion expe-
rience with geotextiles in filtration
applications. Writing on the inno-
vative use of woven monofilament
polyvinylidene chloride and monofila-
ment polypropylene textiles in coastal
structures, Barrett (1966) reported a
wide variety of filters applications that
included drainage systems for vertical
bulkheads and behind seawalls, use as
a bedding material beneath rock-fill
scour protection and breakwater struc-
tures, and placement in combination
with rock fill or concrete-block revet-
ments. In fact, the use of a woven
synthetic “filter cloth” can be traced
back to early product development
in 1958, with innovative applications
in a variety of countries including
the United States, the Caribbean, the
Netherlands and Spain. Reflecting on
this early use, Barrett (1966) identified
the benefits that were found to accrue
from this new type of filter material,
noting in particular:
“There are several advantages... of
plastic filters... common to most types
of structures
• The filtering ability is factory-con-
trolled, and cannot be altered due
to careless placement by labor.
• ... the... filter... has ... tensile
strength...
• Quick, visual inspection assures ...
the filter is in-place, as designed...
• It permits greater opportunity for
consistency in filter design.
• Geographic location and availabil-
ity of materials (sand and gravel)
are eliminated as economic consid-
erations in the design of the filter
system.”
The US Army Corps of Engineers
(USACE) had begun using synthetic
filter cloth in 1962, with recognition
given to two commercially-available
products in 1967, a number that had
increased to about ten products by
1972, the same year in which an
extensive field and laboratory study
reported on material specification and
companion design criteria (Calhoun,
1972). Early applications in Canada
date back to the same time. USACE
fieldwork at five project locations
(involving applications beneath riprap
bank protection and paving block
protection, and around sub-drain col-
lector pipes) established a generally
excellent performance, and confirmed
no significant loss of strength at loca-
tions where the synthetic filter was
buried and therefore not exposed to
UV light. A very extensive compan-
ion laboratory study examined five
woven monofilament products, one
composite monofilament and multifila-
ment woven product, and one compos-
ite needle-punched and heat-bonded
nonwoven product. Two of the woven
monofilament products were identical
to those used at the majority of the
field sites.
The laboratory evaluation of durabil-
ity was conducted with reference to
low-temperature brittleness, UV light
exposure, oxidation and chemical
immersion, with the findings used to
tabulate a series of minimum physical
and chemical requirements of a textile
for use in filtration applications. In
addition, laboratory strength testing
was conducted to determine values
of grab, burst, puncture and abrasion
resistance, with the findings used to
establish three categories of minimum
strength requirement: a category (A)
for severe dynamic loading, associ-
ated with dropping of rip-rap stone at
the time of installation, and continued
abrasive from wave action over the
service life of the structure; an inter-
mediate category (B); and a category
(C) for static loading associated with
wrapping collector pipes and beneath
concrete structures. Most importantly,
filtration compatibility was evaluated
from laboratory permeameter test-
ing with unidirectional flow in order
to investigate factors governing soil
retention and permeability for appli-
cations including revetments with
“relatively high seepage velocities
or rapid fluctuations in the differen-
tial hydrostatic pressures” (Calhoun,
1972). The overall recommendations
addressed criteria for the specification
of synthetic woven textiles, and were
subsequently extended to include addi-
tional criteria for nonwoven geotex-
tiles (USACE, 1977).
At the same time, the question of
material durability and filtration
mechanism was the subject of com-
panion investigations in Europe.
The need for careful evaluation of
minimum strength requirements
was confirmed by the Norwegian
Geotechnical Institute, from labora-
tory permeameter testing of three
heat-bonded nonwoven textiles (NGI,
1974). The Delft Hydraulics Labora-
tory gave specific attention to filtration
compatibility, conducting laboratory
permeameter tests on a variety of 30
woven and nonwoven fabrics (Ogink,
1975). The importance of an intimate
contact between fabric and soil was
emphasized, for which observations
with unidirectional flow established
“a natural... filter had built up under
the fabric” accompanied by an “arch
effect of the grains around the pores in
the fabric”. For conditions of steady
unidirectional flow, experimental find-
ings were used to establish a criterion
for soil retention by a woven fabric
and, separately, a retention criterion
for a nonwoven fabric. Where condi-
tions act to eliminate the combined
benefits of a natural filter and arching,
such as may occur with reversing flow,
then a more conservative soil retention
criterion was recommended along with
recognition of the need for additional
research in support of engineering
practice.
In 1977, a conference was organised
in Paris by the Ecole Nationale des
Ponts at Chaussées and the Labora-
toire Central des Ponts at Chaussées.
It was the first international confer-
ence on the use of fabrics in geotech-
nics, and it had a technical session on
filtration at which McKeande (1977)